1. Field of the Invention
The present invention relates to a highly efficient thermal head which is used for a thermal printer.
2. Description of the Related Art
In a typical conventional thermal head, a glaze heat insulation layer 2 with a thickness of approximately 80 xcexcm is wholly or partially formed on the end of a heat radiation substrate 1 made of alumina or similar material, as shown in FIG. 5.
A convex 2a with a height of approximately 5 xcexcm is formed on the surface of this glaze heat insulation layer 2 by a photolithographic technique.
Also, heating resistors 3 made of Ta2N, Taxe2x80x94SiO2, etc. are formed on the top surface of the glaze heat insulation layer 2 including the ridge-like convex 2a by sputtering and then the heating resistors 3 are processed so as to make up a pattern by a photolithographic technique.
Approximately 2-xcexcm-thick electrodes for supplying an electrical energy to the heating resistors 3 are formed on the top surfaces of the heating resistors 3 by sputtering with Al, Cu, Au, etc.
Then the electrodes are etched by a photolithographic technique to make common electrodes 4 and individual electrodes 5 and external connection terminals (not shown) for the electrodes 4 and 5.
In order to protect the heating resistors 3 and electrodes 4 and 5 against oxidation and abrasion, an abrasion-resistant layer 6 of hard ceramic such as Sixe2x80x94Oxe2x80x94N or Sixe2x80x94Alxe2x80x94Oxe2x80x94N which is resistant to oxidation and abrasion is coated with a thickness of 5 to 10 xcexcm over the heating resistors 3 and electrodes 4 and 5 by sputtering or a similar technique; thus durability in printing is ensured.
This conventional thermal head laminate is bonded to a heat sink 7 composed of an aluminum member, etc. using a resin adhesive 8 in a manner that the heat which is accumulated on the heat radiation substrate 1 during printing may be radiated to the outside; this finished thermal head is mounted into a thermal printer or the like.
In this type of conventional thermal head, Joule heat is generated on the heating resistors 3 to heat heat-sensitive paper or a thermal transfer ink ribbon (not shown) so that characters and images are printed by heat-sensitive paper coloring or ink transfer from the ink ribbon to recording paper such as plain paper.
The recent trend in thermal printers with a conventional thermal head as mentioned above is a compact, lightweight portable model capable of battery-powered operation.
In such a portable thermal printer capable of battery-powered operation, the element which consumes power most is a thermal head since it has a plurality of heating resistors 3.
For the purpose of power saving in a conventional thermal head, the glaze heat insulation layer 2 has been made thicker than before, in order to store more heat.
However, since this conventional thermal head relies only on the approach of increasing the thickness of the glaze insulation layer 2, there may occur an excessive heat accumulation when the printer is run continuously; as a result, when it is used, for example, in a thermal transfer printer, ink from the ink ribbon may be transferred beyond the printing area, causing the phenomenon of trailing in printed image, or a poor print quality.
The present invention is made in view of the above problem and an object of the invention is to provide a thermal head which does not cause deterioration in print quality even in continuous printing or a similar condition and consumes less power than conventional models, and a manufacturing method therefor.
As a first solution to the above problem, the present invention provides a thermal head comprising: a heat insulation layer formed on a top surface of a heat radiation substrate; a plurality of heating elements lined up on a top surface of the heat insulation layer; and an abrasion-resistant layer covering at least the top surfaces of the heating elements, wherein a sacrificial layer of transition metal is formed on a top surface of the heat radiation substrate; a bridge layer of cermet or ceramic material is formed on a top surface of the heat insulation layer including the sacrificial layer; a cavity is made between the bridge layer and the heat insulation layer; a plurality of slits are made in the bridge layer overlying the cavity to expose the cavity; a highly adiabatic inorganic heat insulation layer is formed on a top surface of the bridge layer including the slits; and an inorganic protective layer of a material selected from among silicon or aluminum oxide, nitride and carbide is formed on a top surface of the inorganic heat insulation layer, the heating elements are formed between neighboring ones of the slits over the inorganic heat insulation layer and the inorganic protective layer.
As a second solution to the problem, the heating elements are formed on the inorganic protective layer""s area projecting upward due to the cavity, and the thickness of electrodes is so designed that they are flush with or lower than the heating elements.
As a third solution to the problem, the bridge layer is made of a cermet as a compound of a metal with a high melting point and SiO2 or a ceramic such as SiO2, Si3N4 or Sixe2x80x94Oxe2x80x94N.
As a fourth solution to the problem, the inorganic heat insulation layer is made of a complex oxide or complex nitride as a compound of silicon, transition metal and oxygen or nitrogen, and its thickness is from 5 xcexcm to 20 xcexcm and its thermal diffusivity from 0.3 mm2/sec to 0.4 mm2/sec.
As a fifth solution to the problem, the inorganic protective layer is made of an insulating ceramic such as SiO2, SiC, Sixe2x80x94Alxe2x80x94O, Al2O3 or AlN with a thickness of 0.1 to 1 xcexcm.